Thermistor elements are very useful temperature sensing devices because of their high stablity, repeatability, and large temperature coefficient of resistance so that variations in the thermistor's resistance provide a relatively high sensitive temperature sensing operation. One of the problems in using thermistors, however, is that the relationship between the resistance thereof with temperature in a non-linear one, although such relationship is expressible by a known equation.
In order to make use of thermistors over a wide range of temperature variations it is been found effective to pre-determine the resistance-temperature relationship in accordance with such equation over a relatively wide range of temperatures and then to pre-store such resistance/temperature values in a memory look-up table of a data processing system. During use, a particular temperature can be readily obtained by determining the resistance of the thermistor element and having the data processor obtain the corresponding temperature from the look-up table.
Alternatively, the resistance-temperature equation can be implemented in software in a data processing system and the appropriate resistance supplied thereto so that the temperature can thereupon be computed by the data processing system in accordance with the software implemented equation.
While such data processing approaches provide good accuracy over wide temperature ranges, the output temperature information is provided on a discrete, e.g. non-continuous, basis and the resolution is only as good as the discrete values which are used either in the look-up table or in the software implementation of the equation. Moreover, such techniques require relatively expensive data processing hardware and software to make the necessary computations. Moreover, since the overall resistance/temperature relationship is non-linear, calibration procedures needed in such systems are necessarily more complicated than those needed when implementing linear relationships.
Another approach to making temperature measurements using thermistors is to provide a technique for essentially "linearizing" the relationship between the resistance and the temperature thereof, at least over a limited temperature range. It has been found that by placing the thermistor element in a suitably chosen resistor network, the output temperature response may be made relatively closely linear over a relatively limited temperature range. So long as the temperatures to be measured are confined to that limited range the output from the thermistor circuitry using the resistance network is a linear function of the thermistor resistance. Such an approach can use relatively simple and inexpensive electronic circuitry to convert the resistance measurement to a temperature value so as to provide continuous (non-discrete), analog temperature information. Moreover, such circuitry is relatively easy to calibrate. However, to provide a suitable design accuracy, the temperature range is, by necessity, relatively limited, e.g. such circuitry can provide an accuracy of 0.05.degree. C. only over about a 10.degree. C. range. Moreover, the presence of a resistive network causes the sensitivity of the thermistor to be substantially reduced. For example, the use of a linearizing shunt resistor can cause the sensitivity at the general midpoint of the measurement range to be reduced by as high as 80% with respect to the sensitivity without such a resistance and the signal level at the output can be reduced by almost 60%.
It is desirable to devise an approach utilizing thermistor elements for temperature measurements in which relatively simple circuitry can be used without the need for an expensive hardware and software implementation of the resistance-temperature relationship. Moreover it is desirable that such a technique be devised for providing accurate temperature measurements over a relatively wide range without a great loss in sensitivity.